Currently, heat-dissipating fans commonly used in personal computers include an axial-flow fan, a centrifugal fan and a cross-flow fan. Of these, the most popular one is supposed to be an axial-flow fan.
A fan is primarily consisted of a rotor device and a fan guard arranged beside the rotor device for supporting the rotor device. Referring to FIG. 1, the fan guard 10 of a conventional axial-flow fan is constructed by a main frame 101, a motor holder 102 and a plurality of ribs 103 arranged between the main frame 101 and the motor holder 102. The rotor device 11 includes a motor (not shown) received in the motor holder 102, a shaft ring 111 connected to and driven by the motor to revolve, and a plurality of rotor blades 112 fixed on the circumferential surface of the shaft ring 111 and revolving with the shaft ring 111 to work on the surrounding air to generate an airflow. Through the work of the rotor blades on the surrounding air, the blast pressure is changed from a relatively low value on the air inlet side into a relatively high value on the air outlet side. That is, there is a blast pressure enhancement on the air outlet side.
Unfortunately, when the airflow further flows through the fan guard having the structure as shown in FIG. 1 and as described above, turbulent flows will be generated after the airflow encounters the ribs so as to have an adverse effect on the blast pressure enhancement. Consequently, the efficiency of the fan is reduced.